Ion channels are ubiquitous membrane proteins in the cells of warm-blooded animals such as mammals. Their critical physiological roles include control of the electrical potential across the membrane, mediation of ionic and fluid balance, facilitation of neuromuscular and neuronal transmission, rapid transmembrane signal transduction, and regulation of secretion and contractility.
For example, cardiac ion channels are proteins that reside in the cell membrane and control the electrical activity of cardiac tissue. In response to external stimuli, such as changes in potential across the cell membrane, these ion channels can form a pore through the cell membrane, and allow movement of specific ions into or out of the cell. The integrated behavior of thousands of ion channels in a single cell results in an ionic current, and the integrated behavior of many of these ionic currents makes up the characteristic cardiac action potential.
Arrhythmia is a variation from the normal rhythm of the heart beat and generally represents the end product of abnormal ion-channel structure, number or function. Both atrial arrhythmias and ventricular arrhythmias are known. The major cause of fatalities resulting from cardiac arrhythmias is the subtype of ventricular arrhythmias known as ventricular fibrillation (VF). Conservative estimates indicate that, in the U.S. alone, each year over one million Americans will have a new or recurrent coronary attack (defined as myocardial infarction or fatal coronary heart disease). About 650,000 of these individuals will be first heart attacks and 450,000 of these individuals will be recurrent attacks. About one-third of individuals experiencing these attacks will die as a result. At least 250,000 people a year die of coronary heart disease within 1 hour of the onset of symptoms and before they reach adequate medical aid. These are sudden deaths caused by cardiac arrest, usually resulting from ventricular fibrillation.
Atrial fibrillation (AF) is the most common arrhythmia seen in clinical practice and is a cause of morbidity in many individuals (Pritchett E. L., N. Engl. J. Med. 327(14): 1031 Oct. 1, 1992, discussion 1031-2; Kannel and Wolf, Am. Heart J. 123(1): 264-7 Jan. 1992). The prevalence of AF is likely to increase as the population ages and it is estimated that 3-5% of patients over the age of 60 years have AF (Kannel W. B., Abbot R. D., Savage D. D., McNamara P. M., N. Engl. J. Med. 306(17): 1018-22, 1982; Wolf P. A., Abbot R. D., Kannel W. B., Stroke 22(8): 983-8, 1991). While AF is rarely fatal, it can impair cardiac function and is a major cause of stroke (Hinton R. C., Kistler J. P., Fallon J. T., Friedlich A. L., Fisher C. M., Am. J. Cardiol. 40(4): 509-13, 1977; Wolf P. A., Abbot R. D., Kannel W. B., Arch. Intern. Med. 147(9): 1561-4, 1987; Wolf P. A., Abbot R. D., Kannel W. B., Stroke 22(8): 983-8, 1991; Cabin H. S., Clubb K. S., Hall C., Perlmutter R. A., Feinstein A. R., Am. J. Cardiol. 65(16): 1112-6, 1990).
Antiarrhythmic agents have been developed to prevent or alleviate cardiac arrhythmia. For example, Class I antiarrhythmic compounds have been used to treat supraventricular arrhythmias and ventricular arrhythmias. Treatment of ventricular arrhythmia is very important since such an arrhythmia can be fatal. Serious ventricular arrhythmias (ventricular tachycardia and ventricular fibrillation) occur most often in the presence of myocardial ischemia and/or infarction. Ventricular fibrillation often occurs in the setting of acute myocardial ischemia, before infarction fully develops. At present, there is no satisfactory pharmacotherapy for the treatment and/or prevention of ventricular fibrillation during acute ischemia. In fact, many Class I antiarrhythmic compounds may actually increase mortality in patients who have had a myocardial infarction.
Class Ia, Ic and III antiarrhythmic drugs have been used to convert recent onset AF to sinus rhythm and prevent recurrence of the arrhythmia (Fuch and Podrid, 1992; Nattel S., Hadjis T., Talajic M., Drugs 48(3): 345-71, 1994). However, drug therapy is often limited by adverse effects, including the possibility of increased mortality, and inadequate efficacy (Feld G. K., Circulation 83(6): 2248-50, 1990; Coplen S. E., Antman E. M., Berlin J. A., Hewitt P., Chalmers T. C., Circulation 1991; 83(2): 714 and Circulation 82(4): 1106-16, 1990; Flaker G. C., Blackshear J. L., McBride R., Kronmal R. A., Halperin J. L., Hart R. G., J. Am. Coll. Cardiol 20(3): 527-32, 1992; CAST, N. Engl. J. Med. 321: 406, 1989; Nattel S., Cardiovasc. Res. 37(3): 567-77, 1998). Conversion rates for Class I antiarrhythmics range between 50-90% (Nattel S., Hadjis T., Talajic M., Drugs 48(3): 345-71, 1994; Steinbeck G., Remp T., Hoffmann E., J. Cardiovasc. Electrophysiol. 9 (8 Suppl):S104-8, 1998). Class III antiarrhythmics appear to be more effective for terminating atrial flutter than for AF and are generally regarded as less effective than Class I drugs for terminating of AF (Nattel S., Hadjis T., Talajic M., Drugs 48(3): 345-71, 1994; Capucci A., Aschieri D., Villani G. Q., Drugs Aging 13(1): 51-70, 1998). Examples of such drugs include ibutilide, dofetilide and sotalol. Conversion rates for these drugs range between 30-50% for recent onset AF (Capucci A., Aschieri D., Villani G. Q., Drugs Aging 13(1): 51-70, 1998), and they are also associated with a risk of the induction of Torsades de Pointes ventricular tachyarrhythmias. For ibutilide, the risk of ventricular proarrhythmia is estimated at ˜4.4%, with ˜1.7% of patients requiring cardioversion for refractory ventricular arrhythmias (Kowey P. R., VanderLugt J. T., Luderer J. R., Am. J. Cardiol. 78 (8A): 46-52, 1996). Such events are particularly tragic in the case of AF as this arrhythmia is rarely a fatal in and of itself.
There remains a need in the art to identify new antiarrhythmic treatments, for both ventricular arrhythmias as well as for atrial arrhythmias. The present invention fulfills this need, and further provides other related advantages.